Add a "Select unmasked points" functionality to the metadata browser

[jbudynk]

Running through the Visualizer documentation, I found it would be useful to have a button or option somewhere that lets the user select all the unmasked points. As an example, imagine a user that is playing around with masking/unmasking certain points in their data while fitting the SED. When they're happy with their SED, they want to extract the unmasked points to a new SED to save for later.

A workaround for this may be with the Filter Expressions in the future. One could select the rows whose "Masked" column is unchecked.

Granted, I could see the "select all unmasked points" as a potentially harmful option, as it would be harder to show in the saved dataset that the data comprising the SED was edited; i.e., data provenance is harder to automatically trace in this option. To counteract this, we could make sure that when a SED is serialized (e.g., when saved to disk, or extracted to a new SED), the "masked" information is stored as well.